1. Technical Field
The present invention relates to a technique for controlling an LED printer and, more particularly, to a technique for controlling an LED printer using a separation type LED head.
2. Background Art
A conventional printing process using an LED printer is performed as follows. Flash light emission (exposure) is performed using an LED head opposed to a photosensitive drum based on print data supplied from the outside to form an electrostatic latent image on the photosensitive drum. The electrostatic latent image formed on the photosensitive drum is supplied with toner and developed by a developing device. A toner image formed by developing is transferred by a transfer device onto a sheet fed from a resist roller and deposited thereon.
After that, the toner deposited on the sheet is fixed to the sheet by a fixing device while the sheet is discharged, so printing is completed. Note that the LED printer is controlled by a printing control circuit.
In particular, in the printing process using the LED printer, the sheet is fed by the photosensitive drum provided in a central portion, the resist roller for feeding the sheet, and the fixing device for discharging the sheet. A sheet feeding speed of the resist roller and a sheet feeding speed of the fixing device are each held to a constant value and set such that the sheet feeding speed of the fixing device becomes faster than the sheet feeding speed of the resist roller in order to stably feed the sheet.
Therefore, a phenomenon occurs in which the sheet feeding speed of the sheet is changed among a front end region in which the sheet does not reach the fixing device and thus the sheet is fed by only the resist roller, a central region in which the sheet is fed by both the fixing device and the resist roller, and a rear end region in which the sheet is released from the resist roller and thus the sheet is fed by only the fixing device.
A rotational speed of the photosensitive drum is normally set corresponding to the sheet feeding speed in the central region of the sheet. Therefore, unlike the central region of the sheet, a print length in a sub scanning direction is not normally printed on each of the front end region and the rear end region of the sheet.
For example, as shown in FIG. 9A, when two parallel lines having an interval in the sub scanning direction which is “a2” are to be printed on each of the front end region, the central region, and the rear end region of the sheet, the two parallel lines having the interval “a2” are normally printed on the central region of the sheet.
However, the interval in the sub scanning direction is “a1” in the front end region of the sheet, so the two parallel lines are printed at an interval narrower than the interval “a2”.
In contrast to this, the interval in the sub scanning direction is “a3” in the rear end region of the sheet, so the two parallel lines are printed at an interval wider than the interval “a2”.
Note that in each of the front end region and the rear end region of the sheet, because the rotational speed of the photosensitive drum is different from the sheet feeding speed of the sheet, printing is made with slipping.
Note that the sub scanning direction is a direction in which the sheet is transported and hereinafter, the sub scanning direction is referred to as a direction X. The description will be made based on the assumption that a direction which is orthogonal to the sub scanning direction X and corresponds to an extension direction of a rotating shaft of the photosensitive drum is a main scanning direction Y.
A technique disclosed in Japanese Patent Application Laid-open No. 2001-923226 has been known as a technique for avoiding a change of the print length in the sub scanning direction which is caused by the difference between the rotational speed of the photosensitive drum and the sheet feeding speed.
In the technique disclosed in Japanese Patent Application Laid-open No. 2001-923226, a cycle of exposure to the photosensitive drum is made variable between the front end region and the rear end region of the sheet relative to the central region thereof, thereby correcting the print length in the sub scanning direction in each of the front end region and the rear end region.
Incidentally, in recent years, in order to reduce a cost of an LED head for performing exposure on the photosensitive drum, a separation type LED head which is separated into a plurality of (three to five) LED heads in the main scanning direction or an LED head in which a plurality of (three to five) small-size LED heads are arranged in the main scanning direction has been become mainstream in large-size LED printers.
Such an LED head is hereinafter referred to as a “separation type LED head”. Unlike the separation type LED head, an LED head which is not separated into a plurality of parts in the main scanning direction is referred to as a “single type LED head” and the description will be made.
The separation type LED head includes the small-size LED heads arranged in the main scanning direction, so the respective LED heads are overlapped to be shifted in the sub scanning direction by several mm to several cm.
Reference printing positions of the respective LED heads in the sub scanning direction are different from one another. Therefore, a joint correction circuit is provided to a printing control circuit of the respective LED heads to obtain the same print quality as that of the single type LED head.
The LED printer device described in Japanese Patent Application Laid-open No. 2001-923226 has a structure in which the cycle of exposure to the photosensitive drum is made variable to correct the print length in each of the front end region and the rear end region of the sheet in the case where the single type LED head is used.
However, when the separation type LED head instead of the single type LED head is used in the LED printer device described in Japanese Patent Application Laid-open No. 2001-923226, there is a problem that normal printing is not performed because joint correction timings of the respective LED heads of the separation type LED head which are shifted in position in the sub scanning direction are changed.
In other words, there is a problem that the prints in the main scanning direction are not joined and thus the print length in each of the front end region and the rear end region cannot be normally corrected, thereby reducing printing precision (see FIG. 9B).